1. Field of the Invention
This invention relates to an area cutting method in a numerically controlled machine tool and, more particularly, to an area cutting method for cutting the interior of an area bounded by the curve of an external shape.
2. Description of the Related Art
Types of numerically controlled machining include cutting in which the interior of an area bounded by the curve of an external shape, for example the curve of an external shape comprising straight lines and circular arcs, is hollowed out down to a predetermined depth, and die milling in which the interior of an area is die milled. In such machining, as shown in FIG. 1(A), an area cutting method is conventionally carried out by performing cutting along an (i-1)th cutting path PTi-1 in one direction (the direction of the solid line arrow), moving the tool a predetermined amount at the completion of cutting, then positioning the tool directly above a cutting starting point Ps on the next, or i-th, cutting path PTi, thereafter lowering the tool to the cutting starting point Ps, moving the tool along the i-th cutting path PTi in the direction of the solid line arrow, and subsequently repeating the above unidirectional cutting.
Another area cutting method shown in FIG. 1(B) includes, following completion of cutting along the cutting path PTi-1 of the (i-1)th cutting path, moving the tool from a cutting end point Pe to the cutting starting point Ps on the next, or i-th, cutting path, and thereafter performing cutting along the i-th cutting path PTi. Thus, cutting is performed back and forth reciprocatively in the direction of the arrows.
Still another or third area cutting method shown in FIG. 1(C) includes obtaining offset paths OFC1, OFC2, . . . OFCn offset by predetermined amounts with respect to a curve OLC of an external shape, and moving the tool successively along the offset paths.
However, with the first area cutting method FIG. 1(A) based on unidirectional cutting, the tool must be positioned at the cutting starting point Ps on the i-th cutting path PTi after the completion of cutting along the (i-1)th cutting path PTi-1. This method is disadvantageous in that it results in a long tool travelling distance.
With the second cutting method FIG. 1(B) based on reciprocative cutting, portions are left uncut. In order to cut the uncut portions, the tool must be moved along the external shape curve OLC at completion of the back-and-forth cutting, thereby necessitating both back-and-forth cutting control and cutting control along the shape of the external curve. Accordingly, this method is disadvantageous in that the cutting control is complicated. Further, if an area AR has concavities and convexities, as shown in FIG. 1(D), the second method requires movement for achieving positioning indicated by the dashed lines. This is disadvantageous in that tool travelling distance and cutting time are prolonged. In addition, since the cutting process for the outward trip is different from the cutting process for the return trip, cutting cannot be performed efficiently overall. It should be noted that the cutting processes referred to here indicate up cutting and down cutting processes. FIGS. 2(A) and 2(B) show examples of the down cutting process, and FIGS. 2(C) and 2(D) depict examples of the up cutting process. If the workpiece material has been determined, then a cutting method capable of cutting the workpiece efficiently is selected from the up cutting and down cutting processes. However, with the second method [FIG. 1(B)], the up cutting process [e.g., FIG. 2(A)] and the down cutting process [e.g., FIG. 2(C)] are always mixed, so that cutting cannot be performed efficiently.
With the third method [FIG. 1(C)] of cutting along the offset paths, portions are left uncut at, e.g., the central portion of the area, depending upon the contour of the external shape curve. This method is disadvantageous in that dealing with these uncut portions is a complicated task.